From mboxrd@z Thu Jan 1 00:00:00 1970 From: robin.murphy@arm.com (Robin Murphy) Date: Wed, 7 May 2014 10:51:55 +0100 Subject: [PATCH 0/2] arm: alignment trap tweaks Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi all, Revisiting the alignment trap again thanks to an obscure corner case with NEON alignment hints - patch 1 is a repost from a while back as a ping, because modern userspaces really should just get a resounding "Don't do that!" if they manage to step outside the architecture. Patch 2 is the fix for cases where fixup does need to be turned back on for whatever reason. The following testcase compiled with -marm illustrates the problem: with fixup enabled, when the VLD1 instruction with the alignment hint set faults by performing a misaligned access, it fails to load the NEON registers as expected and eventually writes back nonsense to the base register, resulting in a misleading segfault on the next iteration. --->8--- #include #include static uint8_t buffer[256], got[256]; int main() { uint8_t *base = (uint8_t *)((intptr_t)buffer + 0x7 & ~0x7); void *p = base, *q = got; int i; for (i=0; i<64; i++) base[i] = i; for (i=0; i<4; i++) { asm volatile ("vld1.8 {d0,d1}, [%0]\n vst1.8 {d0,d1}, [%1]\n" : "=r"(p),"=r"(q) : "0"(p),"1"(q) : "d0","d1"); printf("unaligned: %p [%d,%d,%d,...]\n", p++, got[0], got[1], got[2]); } p = base; for (i=0; i<4; i++) { asm volatile ("vld1.8 {d0,d1}, [%0:64]\n vst1.8 {d0,d1}, [%1]\n" : "=r"(p),"=r"(q) : "0"(p),"1"(q) : "d0","d1"); printf("misaligned: %p [%d,%d,%d,...]\n", p++, got[0], got[1], got[2]); } return 0; } --->8--- Robin Murphy (2): arm: SIGBUS on unsupported ARMv6 unaligned accesses arm: don't break misaligned NEON load/store arch/arm/mm/alignment.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) -- 1.7.9.5